1. Field of the Invention
The invention relates to a method for periodically desulphating a nitrogen oxide or sulphur oxide storage vessel of an exhaust emission control system of a multi-cylinder internal combustion engine and to a device for carrying out such a method.
2. Description of Related Art
Exhaust emission control systems having a nitrogen oxide (NO.sub.x) storage vessel, such as a NO.sub.x adsorber catalytic converter, and optionally a sulphur oxide (SO.sub.x) storage vessel which is connected upstream of the latter, such as a so-called SO.sub.x trap, are used in particular in motor vehicles in order to minimize the emission of nitrogen oxide from the internal combustion engine of the motor vehicle. For this purpose, it is known to buffer nitrogen oxide in the NO.sub.x storage vessel in engine operating phases with increased formation of nitrogen oxide, such as lean mode, for example by means of an adsorption process, so that it can be released again and converted in a suitable, later operating phase, such as rich mode, for example by means of an appropriate desorption process and subsequent reduction to form nitrogen oxide. Lean and rich modes are usually to be understood in this context as operation of the engine with a lean, or respectively rich, engine fuel/air ratio, i.e. a fuel/air ratio of the fuel/air mixture combusted in the engine which lies above or below the stoichiometric value.
A known difficulty of such systems consists in the fact that, particularly in the lean mode of the engine, sulphur dioxide is present in the exhaust emission owing to sulphur which is contained in customary fuels and engine oils, which sulphur dioxide can lead, as a result of the formation of sulphates, to sulphur poisoning of the NO.sub.x storage vessel which reduces its NO.sub.x storage capacity. It is therefore known to subject the NO.sub.x storage vessel to a desulphation process whenever there is a perceptible reduction in its NO.sub.x storage capacity, in order to remove the sulphate which has accumulated. Alternatively, the NO.sub.x storage vessel has a SO.sub.x trap upstream of it which is then subjected to a periodic desulphation in an analogous way.
In order to achieve effective desulphation of the nitrogen oxide or sulphur oxide storage vessel, it is known to set increased exhaust emission temperatures of, for example, over 600.degree. C. and a rich storage vessel fuel/air ratio, i.e. one which lies below the stoichiometric value, in which case the term storage vessel fuel/air ratio is to be understood to be the ratio of oxygen or air to fuel and/or non-combusted hydrocarbons in the exhaust emission which is fed to the storage vessel. The setting of such conditions is given, for example, in the publication W. Strehlau et al., New Developments in Lean NO.sub.x Catalysis for Gasoline Fueled Passenger Cars in Europe, SAE 96 2047, 1996.
The German Laid-Open Publication EP 0 636 770 A1 proposes, for the desulphation of a NO.sub.x adsorber catalytic converter, that the internal combustion engine should be reset from a lean engine fuel/air ratio to a rich one, and that when necessary an electric heating device which is assigned to the NO.sub.x adsorber should additionally be activated. The desulphation phase is maintained in each case for a specific time period of approximately 10 minutes.
As a further method in addition to setting a rich engine fuel/air ratio, it has been proposed to make oxygen available in the storage vessel which is to be desulphated, by feeding in secondary air, see the older, non-pre-published German Patent Application 198 02 631.5 (now post-published as Patent DE 198 02 631 C1) and the Patent DE 197 47 222 C1.
The German Laid-Open Publication DE 195 22 165 A1 discloses, in addition to procedures of a different kind, various methods and devices of the type mentioned at the beginning in which, in order to desulphate a NO.sub.x adsorber catalytic converter, at least a subset of the engine cylinders are operated with a rich mixture and the other engine cylinders are operated with one which is leaner in comparison, preferably with a lean engine fuel/air ratio. Insofar as this is possible, this is carried out in that the quantity of fuel for the cylinders which are operated with a lean mixture is reduced while the quantity of air remains constant, while the quantity of air for the cylinders which are operated with a rich mixture is reduced while the quantity of fuel is kept constant. The fuel/air ratio for the cylinders which are operated with a lean mixture here is set larger, by a correction absolute value which can be predefined as a function of the engine operating point, than a desired, predefined overall fuel/air ratio, and the fuel/air ratio for the cylinders which are operated with a rich mixture is set lower, by the same correction absolute value, than the desired overall fuel/air ratio. As an accompanying measure, the ignition time for the cylinders which are operated with a lean mixture is adjusted in the advanced direction and that for the cylinders which are operated with a rich mixture is adjusted in the retarded direction. Subsequently, the intake air quantity is corrected in such a way that a desired engine torque is maintained. In a first variant of the method, a stoichiometric or slightly rich overall fuel/air ratio is set over the entire desulphation period. In a second variant of the method, a stoichiometric overall fuel/air ratio is set during an initial storage vessel heating-up phase, and during a subsequent desulphation mode a rich overall fuel/air ratio is set after a predefinable desulphation temperature has been reached. In both cases, the oxygen which is necessary for the oxidation of reducing agent in order to generate heat is supplied in the exhaust emission by means of the cylinders which are operated with a lean mixture, with the result that the need to supply secondary air is eliminated.